Ceramic multilayer substrates are equipped with semiconductor devices, other electronic components, and the like and used to combine these electronic components with each other by wiring so as to form modules. Ceramic multilayer substrates include a plurality of stacked ceramic insulator layers and wiring conductors in various forms. Wiring conductors include an inner pattern conductor, an outer pattern conductor, an outer electrode, and a via hole conductor. The inner pattern conductor is disposed along a specific interface between ceramic insulator layers inside the ceramic multilayer substrate. The outer pattern conductor is disposed on the outer surface of the ceramic multilayer substrate. The via hole conductor is disposed so as to pass through specific ceramic insulator layers.
In order to make the ceramic multilayer substrate multifunctional, have a higher density, and achieve higher performance, it is effective to arrange the above-described wiring conductors in high density. Incidentally, a firing step is necessary for producing the ceramic multilayer substrate. However, during such a firing step, shrinkage of a ceramic material occurs due to sintering. The sintering shrinkage tends to occur unevenly in the entire ceramic multilayer substrate. Therefore, undesirable deformation or distortion may be caused in a wiring conductor. The occurrence of such deformation or distortion in the wiring conductor hinders the wiring conductors from being arranged in high density.
Then, it has been proposed to apply a so-called non-shrinkage process in which the occurrence of shrinkage in a principal surface direction of the ceramic multilayer substrate during a firing step can be substantially prevented when the ceramic multilayer substrate is produced.
In a method for manufacturing a ceramic multilayer substrate by the non-shrinkage process, a low-temperature sintering ceramic material that can be sintered at a temperature of, for example, 1,000° C. or lower is prepared and, in addition, an inorganic material powder that is not sintered at the sintering temperature of the above-described low-temperature sintering ceramic material and that performs the function of suppressing shrinkage is prepared.
Subsequently, a plurality of green sheets of base-member ceramic layers containing the low-temperature sintering ceramic material and green sheets of constraining layers containing the inorganic material powder that suppresses shrinkage are stacked, and wiring conductors related to the base-member ceramic layers are disposed so as to produce a multilayer body before firing, where the multilayer body after firing serves as a ceramic multilayer substrate. In this regard, the base-member ceramic layer is a ceramic layer that contributes to realization of the electrical characteristics of an insulator ceramic layer.
The multilayer body before firing, produced as described above, is fired. The multilayer body before firing has the above-described structure. Therefore, the base-member ceramic layer shrinks substantially in the thickness direction during the firing step, and shrinkage in the principal surface direction is suppressed. As a result, in the ceramic multilayer substrate produced by firing the multilayer body having the above-described structure before firing, uneven deformation does not easily occur.
Consequently, undesirable deformation or distortion does not occur easily in the wiring conductors, and the wiring conductors can be arranged in high density. Japanese Unexamined Patent Application Publication No. 2002-368421 (Patent Document 1) proposes an example of such a ceramic multilayer substrate.
FIG. 15 is a sectional view showing a ceramic multilayer substrate 200 described in Patent Document 1. The ceramic multilayer substrate 200 includes base-member ceramic layers 201, constraining layers 202 which are disposed in contact with the respective principal surfaces of specific base-member ceramic layers 201, inner pattern conductors 204, outer pattern conductors 205, and via conductors 207. The via conductors 207 pass through specific base-member ceramic layers 201, and connect specific inner pattern conductors 204 to each other or connect a specific inner pattern conductor 204 to an outer pattern conductor 205.
In this regard, as described above, the base-member ceramic layer 201 is composed of the low-temperature sintering ceramic material that can be sintered at a temperature of, for example, 1,000° C. or lower, and the constraining layer 202 is composed of a ceramic material that is not sintered at the sintering temperature of the above-described low-temperature sintering ceramic material.
In the ceramic multilayer substrate 200, when the base-member ceramic layer 201 is sintered, shrinkage in the principal surface direction is suppressed by the constraining layers 202. As a result, uneven deformation or distortion does not easily occur in the ceramic multilayer substrate 200. Consequently, it is mentioned that undesirable deformation or distortion does not occur easily in the wiring conductor, and the wiring conductors can be arranged in high density.